Modern electronic devices such as a notebook computer comprise a variety of memories to store information. Memory circuits include two major categories. One is volatile memories; the other is non-volatile memories. Volatile memories include random access memory (RAM), which can be further divided into two sub-categories, static random access memory (SRAM) and dynamic random access memory (DRAM). Both SRAM and DRAM are volatile because they will lose the information they store when they are not powered. On the other hand, non-volatile memories can keep data stored on them. Non-volatile memories include a variety of sub-categories, such as read-only-memory (ROM), electrically erasable programmable read-only memory (EEPROM) and flash memory.
Flash memory is a non-volatile device that can be electrically erased and reprogrammed. A typical flash memory comprises a memory array having a large number of flash memory cells arranged in rows, columns, and blocks. One of the most commonly known flash memories is the one-transistor flash memory. The memory cell of the one-transistor flash memory is fabricated as a field-effect transistor having two gates, namely a control gate and a floating gate. The floating gate is capable of holding charges and is separated from source and drain regions.
Each of the memory cells can be electrically charged by injecting hot electrons across an oxide layer (tunneling layer) onto the floating gate. The charges can be removed from the floating gate by tunneling the electrons to the substrate through the tunneling layer during an erase operation. Thus the data in a memory cell is determined by the presence or absence of charges in the floating gate.
As technologies evolve, semiconductor process nodes have been scaled down for high density flash memory integrated circuits. As a result, the form factor of flash memory devices has been improved from shrinking the semiconductor process node (e.g., shrink the process node towards the sub-20 nm node). As semiconductor devices are scaled down, new techniques are needed to maintain the electronic components' performance from one generation to the next. For example, poor or reduced cycling and data retention capabilities are two major concerns in the tunnel oxide when the tunnel oxide traps more electrons than desired during program erase operations. Such undesirable trapping of electrons makes the overall flash memory device less efficient.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.